Prior art systems for electromagnetically transmitting amplitude modulated signals into the body of a patient include a transmitter having a tuned circuit which is coupled closely to a tuned circuit provided by the receiver portion of the system that is implanted in the body. FIG. 1 of the drawing is a schematic of a prior art receiver which serves to receive the transmitted AM signal and detect the modulating signal for application to the electrodes which connect with the portion of the body to be stimulated. The prior art receiver of FIG. 1 includes a tuned circuit portion and a demodulator portion. The tuned circuit portion includes a coil 10 and a tuning capacitance, which can be provided by one or more capacitors, and is depicted by the single capacitor 12. The demodulator portion which is connected across the capacitor 12 includes a rectifier that is provided by a semiconductor diode 14 which has its anode connected to one side of the capacitor 12. A resistor 16 and a parallel connected capacitor 18 are connected between the cathode of diode 14 and the other side of the capacitor 12. A capacitor 19 is provided between the cathode of diode 14 and one output conductor 20 for the receiver. Capacitor 19 serves to pass the modulating signal and block the passage of direct current to the output of the demodulator portion. A second output conductor 22 for the receiver is connected to the side of the capacitor 12 that is not connected to the diode 14. An AM signal received by the tuned circuit portion is demodulated by the demodulator portion wherein the AM signal is passed by the diode 14 with capacitor 18 providing decoupling of the carrier portion of the modulated signal. Resistor 16 provides a direct current path to the second output conductor 22 side of the receiver for limited discharge of capacitors 18 and 19 during the time the carrier signal is decreasing in magnitude. A signal which varies in accordance with the modulating signal for the AM signal is thus presented to the load depicted by resistance 21 which is provided by the portion of the body of the patient that is to be connected between the two output conductors 20 and 22.
This prior art receiver is not very efficient in that only about ten percent (10%) of the total power received by a circuit is passed on to the load 21 connected to the two output conductors 20 and 22. The remainder of the power received is dissipated in the resistor 16. In the case of systems using a transmitter that is powered by batteries, it is apparent that an improvement in the efficiency of the prior art receiver is desirable since it will allow the transmitter to be operated at a lower power level for a desired output from the receiver to extend the operating life of the transmitter batteries. Limited improvement can be achieved by selection of the resistance value of the resistor 16 since it must be properly matched to the load impedance. If resistor 16 presents too high a resistance clipping of the waveform occurs and if its resistance is too small, the power loss will be excessive.